This invention relates to flame retardant additives for polymers, and more particularly to a method for preparing brominated polymers useful as flame retardants.
The use of brominated polymers, especially polycarbonates, as flame retardant additives for other polymers has been known for some time. For example, U.S. Pat. Nos. 3,915,926 and 3,936,400 describe the preparation of polycarbonates derived from such brominated monomers as 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane, hereinafter sometimes designated "TBBPA", and the use of said polycarbonates as flame retardants for polyesters, such as poly(ethylene terephthalate) and poly(butylene terephthalate). Typically, the polymer employed is a TBBPA homopolymer or a copolymer of TBBPA with 2,2-bis(4-hydroxyphenyl)propane, hereinafter sometimes "bisphenol A". For the most part, it is an oligomer having a weight average molecular weight in the range of about 1,500-5,000, all weight average molecular weights herein being determined by gel permeation chromatography relative to polystyrene, since high molecular weight TBBPA polycarbonates tend to produce such high viscosities and glass transition temperatures in the flame retardant blend that processing is difficult or impossible.
Several disadvantages are encountered in the use of such TBBPA oligopolycarbonates as flame retardants. First, they are often expensive to manufacture, particularly since their preparation may require redirection of an existing polycarbonate manufacturing facility with a resulting loss in production of conventional polymers such as bisphenol A polycarbonates. Second, the molecular weights of the blends in which the TBBPA oligopolycarbonates are incorporated tend to change markedly when they are exposed to thermal aging in air. A typical TBBPA copolymer, for example, doubles the molecular weight of a base poly(1,4-butylene terephthalate) in which it is incorporated after about a day at 190.degree. C., and the blend then loses molecular weight.
U.S. Pat. Nos. 5,021,521 and 5,414,057 describe the preparation of various types of polycarbonates by equilibration or redistribution. In such reactions, a high molecular weight polycarbonate is heated in the presence of a catalyst, and optionally a diaryl carbonate or di- or polyhydroxyaromatic compound, to produce a redistributed or equilibrated polycarbonate of a different, usually lower, molecular weight.
The catalysts employed according to said patents include a wide variety of chemical compounds, chiefly bases and Lewis acids. A strictly non-exhaustive list of examples includes alkali and alkaline earth metal carboxylates such as lithium stearate, hydrides such as sodium hydride, borohydrides such as sodium borohydride, organotin compounds such as di(n-butyl)tin oxide, titanate esters such as tetra-n-propyl titanate, and tetraalkylammonium compounds such as tetramethylammonium hydroxide, tetramethylammonium acetate and tetra-n-butylammonium tetraphenylborate. Copending, commonly owned application Ser. No. 08/768,871 discloses hexaalkylguanidinium bisphenolates which are also useful as catalysts in a wide variety of equilibration reactions of this type.
Such an equilibration reaction (the word "equilibration" being used hereinafter for brevity to designate both equilibration and redistribution reactions) might be expected to be useful for producing brominated polycarbonates having suitable viscosity properties for use as flame retardants. It has been discovered, however, that many of the previously disclosed equilibration catalysts are not effective in the presence of brominated polycarbonates. This is true even though said catalysts are effective for equilibration of other polycarbonates, such as bisphenol A homopolycarbonates.
It is of interest, therefore, to develop a method of preparing brominated polycarbonates having high stability under thermal air aging conditions and having viscosities which are a good match for the polymers in which they are to be incorporated as flame retardant additives. It is further of interest to prepare such polymers by a relatively simple method such as equilibration.